The present invention relates to a driving power unit for a piezoactuator in which, for example, a lamination-type piezoelement is used as a driving element.
Conventionally, a DC voltage source supplies a voltage equal to that applied to a piezoactuator. With this system, the applied voltage is turned on and off by means of a contact switch circuit or a non-contact switch circuit, and is discharged via a discharge circuit. In addition, when two or more voltage levels are needed, each voltage must be supplied by a separate power source or the voltage is divided in a resistance-type voltage dividing circuit and the divided voltages are switched using a switching circuit.
FIG. 2 shows a conventional example with an AC power source 201, a slideac 202, a step-up transformer 203, a bridge with 204 diodes, a smoothing capacitor 208, a discharging switch 209, charging switches 210 and 211, a limiting resistance 212, a lamination-type piezoactuator (PZT) 213 and a power supply line 214 from another power source.
Since the piezoactuator 213 is also a capacitor in a conventional power source as described above, it is necessary to insert the limiting resistance 212 to limit current surges in the voltage application circuit. For this reason, when the piezoactuator 213 approaches the power source voltage, the current flowing into the piezoactuator is reduced because of the limiting resistance thereby decreasing the boosting speed of the piezoactuator. This delays the voltage buildup. In addition, a switching element that can withstand large current surges is needed to improve the buildup by decreasing the limiting resistance. This has also proved useless, however, because almost no current actually flows after the power source voltage is reached because the piezoelement also acts as a capacitor.
A discharging circuit is also required in addition to the voltage application circuit. A high voltage is needed for a high power piezoelement as used in a piezopress (a press in which a piezoelement is used) necessitating use of an expensive contact relay. In this case, if the limiting resistance is reduced, it may not be possible to interrupt the circuit if the piezoelement short-circuits between lamination electrodes.
A purpose of the present invention is to provide a driving power unit for a piezoactuator which solves the problems described above.